1. Field of the Invention
The present invention relates to an optical disk which includes a plurality of information record layers, and allows information to be recorded/reproduced on/from the information record layers with a laser beam.
2. Description of the Related Art
In recent years, information in optical disks has been recorded at a higher density. As a result, pits or grooves formed in optical disks have been required to be formed at a higher density. Furthermore, e.g., Jpn. Pat. Appln. KOKAI Publication No. 9-138970 discloses a multi-layer optical disk which has a plurality of layers having pits or grooves, and allows information to be optically read from one side of the optical disk.
For example, a dual-layer optical disk is provided as the multi-layer optical disk. One of the both sides of the dual-layer optical disk has a first substrate on which a first information record layer is formed, and the other has a second substrate on which a second information record layer is formed. The first and second information record layers are provided opposite to each other, and bonded to each other by a transparent ultraviolet curing resin. Each of the above information record layers is a phase change record layer which enables information to be recorded therein by utilizing a phase change occurring due to irradiation of a laser beam, or a read-only reflection layer having a surface which is formed to have pits representing information.
By way of example, a conventional method of bonding substrates by performing spin-coating will be explained.
In the conventional method, ultraviolet curing type resin is dropped onto an information record layer of a substrate such that it is ring-shaped coaxial with the substrate. It is spun for a predetermined time period to coat the substrate such that the surplus part of the ultraviolet curing type resin is removed from the substrate, as a result of which the ultraviolet curing type resin is provided to have a predetermined thickness. After preparing two substrate coated with ultraviolet currying type resin in such a manner, the following steps are carried out: the surfaces of the substrates which are coated with the ultraviolet curing type resin are made to face each other; the centers of the substrates are put together by using a center pin; the substrates are bonded together in a vacuum; and ultraviolet rays are radiated onto the substrates for a predetermined time period, thereby curing the ultraviolet curing type resin. As a result, a spacer layer (adhesive layer) having a predetermined thickness is provided.
However, when a spacer layer is formed by the above spin-coating method, part of the spacer layer which is located on an inner circumferential part of a disk tends to be thin, since the ultraviolet curing type resin has viscosity and the position onto which the resin can be dropped is limited to a specific position. In dual-layer DVD-ROMs currently on the market, a problem such as a malfunction does not occur, even when the spacer layer on the inner and outer circumferential portions of the disk is uneven in thickness. This is because an optical pickup is optimally designed.
In recent years, dual-layer optical disks containing information recorded at a higher density has been made, by adopting a method of increasing the NA of a laser beam for recording/reproducing to decrease the diameter of the laser beam. As such a dual-layer optical disk, if a dual-layer optical disk including a spacer layer having a non-uniform thickness is adopted, a serious problem such as a focus error or crosstalk occurring between layers arises. Consequently, the recording/reproducing characteristics and reliability of the optical disk remarkably lowers.
In order to the above problems, the following method is tried: a spacer layer is formed of an adhesive sheet having a uniform thickness such that the spacer layer is uniformly formed in thickness over the entire surface of a dual-layer optical disk. However, this method requires a new step of removing a separator attached to the adhesive sheet, during a step of bonding substrates together. Furthermore, if the step of removing the separator from the adhesive sheet is carried out for a short time period, there is a strong possibility that the record layer may be damaged by a mechanical stress acting when the separator is removed from the adhesive sheet. On the other hand, if the step of removing the separator is carried out for a long time period, the productivity of optical disks lowers.
Furthermore, it can be considered that in manufacturing light-transmissivesive substrates, the thicknesses thereof are strictly managed to reduce the variance in thickness between the transmissive-light substrates, thereby compensating for the unevenness of the spacer. However, this method also lowers the yield, and remarkably lower the productivity.